This is an investigation of metabolic processes regionally and in vivo by labeling metabolic substrates with cyclotron-produced radioactive isotopes (mostly O15, C11, and N13) and then following the fate of the label with externally placed radiation detectors. A computer interfaced multiprobe scintillation counter system for the study of cerebral hemodynamics and metabolism has been constructed and is being tested. A devise consisting of 24 scintillation counter probes distributed around a patient's head to obtain transverse tomograms showing the distribution of nuclides in the brain is being developed. The investigation of cerebral hemodynamics and metabolism carried out in animals and patients with cerebral pathology employ regional brain measurements of blood flow, oxygen metabolism, blood volume, glucose metabolism, metabolism of acetoacetate in patients on ketogenic diets, the fate of intravenously injected ammonia, and the measure of tissue pH by means of C11-labeled bicarbonate. Labeling metabolites with N13, C11, F18, Br77, and I123 is investigated. The short-lived isotopes will be used to label biologically important molecules (urea, chloresterol) and recoil iodination methods developed by use for use with I123 will be extended to Br77.